Voltage Divider Calculator — With Optional Load

Find the voltage at the junction of two series resistors and see how a resistive load changes it. The upper resistor connects the input source to the junction; the lower resistor connects that junction to ground. The calculator also reports source current and the divider’s unloaded output resistance.

How to use this tool

1. Enter the source voltage relative to ground.
2. Enter R1 above the output and R2 below it.
3. Select a common resistance unit.
4. Leave the load blank for an unloaded divider, or enter the resistance connected across R2.

Formula

Unloaded: Vout = Vin × R2/(R1 + R2); loaded: replace R2 with R2 || Rload

The parallel combination of two positive resistances is their product divided by their sum. The numerical implementation uses normalized ratios to avoid unnecessary overflow. Output resistance is R1 || R2 with an ideal input voltage source set to zero.

Example

With 12 V, R1 = 1 kΩ and R2 = 2 kΩ, the unloaded output is 8 V and the input current is 4 mA. Adding a 2 kΩ load makes the lower equivalent resistance 1 kΩ, reducing the output to 6 V and increasing input current to 6 mA.